Study On Mechanical Behavior Of Soldier Piles Caused By Excavation Of Subway Foundation Pits In Soft Soil Area

The retaining structure of foundation pit is used to ensure the stable deformation after unloading the soil,the soldier piles play an important role which transfer the vertical load from the supporting system.There are few studies to discuss the influence of the internal force and displacement about the soldier pile.During the excavation of the subway foundation pit,there are many factors which affect the uplift and settlement of the soldier pile,especially in engineering and hydrogeology conditions,several exterior-protected structures of foundation pit,construction sequence and so on.These factors will result in the different variation trend of soldier piles.The uplift and settlement of soldier pile will cause the supporting system and other retaining structure in an out-of-plane stress state,and the joints may cause some complicated state of stress with the supporting system and retaining structure,the component is subjected to bending,shear,torsion and the combined stress state.Some engineering cases show that the unstable failure of joint affects safety of foundation pit directly.At present,the excavation of subway foundation pit is a hot topic in geotechnical engineering.The research about the uplift and settlement of the pile is mainly focused on the analysis of measured data and theoretical analysis,but different soil layers and engineering environment will cause great differences.Therefore,this subject is based on subway foundation pits in soft soil area.On the basis of site measurement,this subject will combine with the research about mechanical behavior caused by excavation of foundation pit,and according to the relationship of skin friction between the pile and the surrounding soil,the mechanical mechanism and properties of the uplift and settlement of the soldier piles are analyzed,and it needs to establish the finite element model of the uplift and settlement of the soldier pile about excavation of subway foundation pits,and this paper will give some conclusions about the space-time regulations of the uplift and settlement of the soldier piles during excavation of subway foundation pit.The main contents and conclusions concluded from this paper are showed as below:1、This subject puts the subway foundation pit in soft soil area as the research background,which combining with the hydrological and geological conditions,this paper analyzes the space-time regulations of the uplift and settlement of the soldier piles during excavation of subway foundation pit,and the uplift and settlement of the soldier piles represents various effects with the time.The trend of the uplift and settlement of the soldier piles develops rapidly when the last layer of soil is excavated.The subway foundation pit has the spacial dimension effect,the increasing rateof uplift and settlement of the soldier pile at the center of the foundation pit is more than the soldier pile in the edge of foundation pit.There is a normalized rule for the uplift and settlement of the pile.The uplift and settlement of the soldier piles in the center of the foundation pit divided by the square of the excavation depth is 0.020 ‰,however the value in the edge of foundation pit is about0.012 ‰.2、The uplift and settlement of the soldier piles are uneven.It will result in differential-settlement between the soldier piles,the maximum differential-settlement is located in the transition zone between work well and the central area of foundation pit,the range of variation is from 25.3% to 37.4% of the length of foundation pit.3、The soldier pile and retaining structure concurrently show the different measured value.The differential-settlement between soldier piles and diaphragm walls is approximately linear with the increase of excavation depth.The differential-settlement between soldier piles and diaphragm walls reaches that the maximum value is 1/1000 of the depth of the foundation pits.4、According to the stress of the soldier pile.The relationship between the vertical displacement trend of the soldier pile and the depth of surrounding soil is established,this paper puts forward the model to calculate the heaving of soldier piles from the former research about heaving of soldier piles.The results show that the measured uplift and settlement of the pile is less than the calculated value of the model.5、According to Technical Code for Monitoring of Building Foundation Pit Engineering in Shanghai,the calculation method about control value of pile is put forward according to the ultimate moment of reinforced concrete support.6、By using the finite element software PLAXIS,this modle will take Hardening Soil model with small-strain stiffness,and the reasonable model parameters are selected.The rationality of the finite element model is verified by comparing the numerical model with the measured data.7、The numerical model of uplift and settlement is analyzed by changing different sensitive factors.The results show that the uplift of soldier pile in foundation pit can be obviously reduced when the insertion ratio of the soldier pile is 1: 1.5;When the reinforcement depth of the passive area reaches 3m,the uplift and settlement of the pile increases slowly.8、The deflection of reinforced concrete support is changed due to the uplift and settlement of soldier piles,and the bending moment of reinforced concrete support changes with it.Themaximum position and variation amount of bending moment are discussed.According to the bending moment of reinforced concrete support and the linear fitting curve,the value of bending moment is very close to the linear fitting curve,and the difference of value is 5.7%.